The use of fiber-optic devices for remote viewing and photography in medical and industrial applications requires efficient transmission of light through a limited number of optical fibers..sup.1 Limitations inherent in the imaging of conventional incoherent light sources onto small diameter fiber-optic bundles require the use of high-power sources such as arc lamps and result in undue increases in the size of the device. It has been shown that white light from a multiwavelength ion laser can be focused onto a single 85 .mu.m optical fiber to provide adequate illumination for viewing and color photography..sup.2 Such a dramatic reduction in the number of required illumination fibers is of particular importance in medical instrumentation, allowing for either a significant reduction in the size of the instrument or a corresponding increase in the size of the ancillary channel to accommodate a larger surgical tool. FNT .sup.1. M. Epstein, Opt. Eng., 13, 139 (1974). FNT .sup.2. M. Epstein and M. E. Marhic, Proc. IEEE (Lett), 63,727 (1975).
The need for highly flexible optical fibers for imaging and illumination suggests the use of plastic optical fibers. In addition, unlike fibers made of glass, the plastic fibers do not require protective shielding when employed in medical instruments. In most applications, so far, plastic fibers have been used mainly for illumination; e.g., as light couplers in electro-optics or in display and decorative devices. The plastic optical fiber, not unlike its step-index glass counterpart, is made of two materials; e.g., in one type of optical fiber the core is made of polystyrene and the cladding of Lucite (poly-methylmethacrylate) while in another Lucite is used for the core and poly (fluoroalkyl methacrylate) for the cladding. The fabrication of aligned plastic multifibers for imaging has been, so far, quite difficult mainly due to lack of raw materials with adequate mechanical properties; i.e., appropriately clad rods which can be pulled in a furnace to very thin fibers without breakage. Flexible plastic fiber-optic imaging structures have been constructed using an assembly of 10 .times. 10 arrays of fibers 250 .mu.m square with each fiber 25 .mu.m in diameter and have been used to construct a flexible laryngoscope for endotracheal intubation..sup.3 A single multifiber containing a large number of aligned plastic fibers has been fabricated with single fiber diameter down to 10 .mu.m..sup.4 While further improvements in the fabrication of high-resolution plastic fiber-optic imaging structure can be expected, the selective transmission of light through such fibers limits their use for both imaging and illumination. White light transmitted through a plastic optical fiber appears yellow and, thus, distorts the reflected color image, a most objectionable feature, in particular, in medical applications. The use of synthesized light in a multiwavelength laser, however, allows for a convenient technique of compensation of the source, such that the reflected image appears as if illuminated with an undistorted white light. Of course, such compensation depends on the number of wavelengths of light used in the source, and, in the usual case of three colors, can, though quite adequately, be only approximated. FNT .sup.3. C. M. Stiles, Q. R. Stiles, and J. S. Denson, JAMA, 221,1246 (1972). FNT .sup.4. Made by Welch Allyn, Inc. Skaneateles Falls, N.Y. 13153.
When illuminated by a conventional white light source, such as in presently commercially available illuminators, the output of a plastic fiber-optic bundle appears yellow. FIG. 1 shows the transmittance of an optical step fiber consisting of a polystyrene core and Lucite cladding. The measurement was obtained with a Zeiss MM12 spectrophotometer using an aligned bundle of plastic fibers 66 cm long and 1 mm in diameter..sup.4 FNT .sup.4. Made by Welch Allyn, Inc. Skaneateles Falls, N.Y. 13153.
In order to evaluate the color of the transmitted light, the C.I.E. chromaticity coordinates (trichromatic coefficients) were calculated..sup.5 Radiant power at different wavelengths was measured using an Oriel Model 7240 monochromator and a Coherent Radiation Model 212 power meter. The computed power, corrected for the response of the power meter and the monochromator, was multiplied by the values of the C.I.E. tristimulus functions x, y, and z for the corresponding wavelengths. Summing the results for all wavelengths yields the three tristimulus values EQU X = .SIGMA.E.sub..lambda. x.sub..lambda., Y = .SIGMA.E.sub..lambda. y.sub..lambda., Z = .SIGMA.E.sub..lambda. z.sub..lambda., (1)
and the C.I.E. chromaticity coordinates can then be computed to yield EQU x = X/(X+Y+Z) and y = Y/(X+Y+Z). (2)
the chromaticity coordinates for light emanating from the plastic fiber-optic bundle described above when illuminated with a quartz iodine tungsten projector lamp (ACM1 BLS97) fiber-optic illuminator) were found to be x = 0.5 and y = 0.45 with data points obtained every 10 nm apart. These coordinates correspond to the color of gold as shown by point A on the chromaticity diagram in FIG. 2. A similar measurement using an arc lamp source (Marc 300 in an ACMI FCB-1000 fiber-optic high-intensity illuminator) yielded an output light given by point D in FIG. 2, which corresponds to a pale yellow. It should be noted that the temperature at the input to the fiber-optic plastic bundle, even though coupled through another glass fiber-optic bundle, was high enough to melt the former. FNT .sup.5. R. S. Hunter, The Measurement of Appearance, (Wiley-Interscience, New York, 1975), p.91.
In order to correct for the distorted color transmission through plastic fibers by means of filters, substantial reduction in total power is required. Thus, to achieve an output at a nearby white which is a warm white given by the chromaticity coordinates x = 0.42 and y = 0.4, point E in FIG. 2, requires a substantial reduction in overall radiant output power. For example, in the case of the projector-lamp source described above, a warm white light output can be achieved by decreasing the radiant power at the wavelengths from 510 nm and above by 75 percent, which results in an overall power reduction of 70 percent, or by reducing the output power above 560 nm and from 530 to 560 nm by 75 and 100 percent, respectively. Adding to the above the losses associated with focusing of incoherent sources onto fiber-optic bundles of very small diameter, and the need to use absorption filters at high light intensities, makes the use of conventional light sources impractical.
The use of synthesized white laser light provides a convenient solution to both the color balance and focusing problems. The ability to focus coherent light onto a very small diameter optical fiber allows for a very efficient use of the light source, i.e. nearly all of the radiant power generated can be applied to the fiber using a simple and inexpensive lens..sup.2 The white light is synthesized either by using three separate monochromatic lasers at the different colors, a multiwavelength laser or a combination of both. FNT .sup.2. M. Epstein and M. E. Marhic, Proc. IEEE (Lett), 63,727 (1975).
It is a principal object of the present invention to synthesize white laser light more efficiently than has been done using prior art methods. By decreasing power requirements of the laser it is possible to use an air cooling system rather than water cooling, and the laser can be produced more compactly and more inexpensively.
It is a related object of the present invention to provide a time multiplexing system for successively lasing at a plurality of discrete wavelengths, thereby eliminating competition among the various discrete wavelengths. By eliminating competition among wavelengths, lasers constructed in accordance with the teachings of the present invention may utilize many materials such as He-Se, He-I.sub.2, and He-Ne-Cd-Se which have not heretofore been practical as lasing gases.
It is another object of the present invention to provide a method for selectively varying the color of light produced by a laser, without competition among discrete wavelengths. By producing laser light having an increased proportion of blue relative to red and green, such laser light will appear white after transmission through plastic optical fibers. The white laser light thereby synthesized is suitable for illuminating objects where color photographs are desired.
Yet another object of the invention is to eliminate scintillation effects which ordinarily accompany simultaneous lasing at a plurality of discrete wavelengths.
Additional objects and advantages of the present invention will become apparent from the following specification, taken in conjunction with the drawings.